An integrated method is provided for improving the production rate of biogas. The method uses lignocellulosic depolymerization with anaerobic digestion. The arrangement and combination of the structure and composition of a lig-nocellulosic material are destroyed through a pretreatment of acid-catalytic steam explosion or steam explosion depolymerization. The period of anaerobic digestion is shortened and biogas production is enhanced. The present inven-tion effectively improves the biogas conversion yield of the lignocellulosic ma-terial. Through the depolymerization pretreatment under a specific pH and operation condition, the lignocellulosic material (e.g. residual agroforestry bi-omass, energy crop, etc.) is converted into a depolymerized material. Then, biogas is produced through anaerobic digestion as a source of gaseous fuel supplied for biomass power or transportation fuel.